In this paper, we used Mendelian randomization to investigate the association between testosterone and IBD in men. Specifically, a two-sample MR was used. We demonstrated that genetically predicted T concentrations were positively associated with IBD risk. In short, high levels of testosterone are a risk factor for IBD.
Considerable observational evidence suggests that T is an important factor influencing a wide range of diseases. T levels in humans are associated with the risk of coronary heart disease (24), type 2 diabetes mellitus (25), asthma (26), and polycystic ovary syndrome (PCOS)(27). The above results are sufficient to suggest that T is an important factor in the development and progression of this disease, but further studies are needed to determine whether T affects this disease, as is the case for IBD. To the best of our knowledge, no previous study has investigated the association between testosterone (T) and IBD at the genetic level.
T was positively correlated with IBD, which was inconsistent with the findings of some observational studies. For example, Ahmad et al. reported that testosterone administration to elderly hypogonadal men improved their Crohn's disease activity index. Therefore, testosterone may reduce chronic inflammation in the intestinal wall in patients with Crohn's disease through an immunosuppressive effect. These results indicated that testosterone levels were negatively correlated with CD activity (28). In contrast, a study by Adi et al. revealed that prostate cancer patients receiving androgen deprivation therapy (ADT) had a significantly lower risk of developing UC (29). Testosterone levels were positively correlated with having UC, which was in line with our findings. The reasons for these two opposite results may involve dysregulation of the body's gut microbiota, hyperimmunity, and the inflammatory response. Based on the results of the present study, it may be more meaningful to explore the causal effects of T on IBD.
As the largest immune organ of the body, the gastrointestinal tract carries the balance of the intestinal microbiota, which is the basis for maintaining normal immune function and inflammatory balance in the body. Numerous studies have collectively concluded that sex differences exist in the human gastrointestinal microbiota (30, 31). The interaction between sex hormones and the gut microbiota is associated with the development of many diseases in the body, including polycystic ovary syndrome, type I diabetes, and postmenopausal osteoporosis (32). An animal study demonstrated that the gut microbiota of mice with testosterone-induced polycystic ovary syndrome was also altered (33) and that testosterone had a strong and long-lasting effect on gut microbiota homeostasis (34). In addition, denuded mice have been shown to affect the gut flora because the gut microbiota is affected by a reduction in testosterone, leading to dysbiosis of the gut microbiota (35). 6-a Dihydrotestosterone (DHA) has also been shown to decrease gut microbial diversity and is thought to play an important role in maintaining gut microbial homeostasis (36). The gut microbiota in turn regulates testosterone secretion and catabolism (37). This suggested a bidirectional regulatory relationship between the gut microbiota and testosterone.
The powerful effect of the gut microbiota on the body's immunity indicates that testosterone levels also affect the body's immunity and inflammation. Testosterone has been proven to have an immunomodulatory effect, suppressing the immune function of the body (38). The mechanism by which testosterone regulates the body's immune function has not been fully clarified. Some studies have shown that testosterone can reduce the production of proinflammatory factors such as TNF-a, IL-1β, and IL-6 (39) produced by human macrophages, monocytes, and endothelial cells, while the production of anti-inflammatory factors, such as IL-10 (40), increases. Moreover, testosterone inhibited the proliferation and differentiation of lymphocytes (41). In addition, testosterone also reduces the expression of TLR4 receptors (42) in macrophages, which has been shown to influence the immune function of the body (43). Moreover, testosterone affects lipid mediators (LMs), which are produced by neutrophils, monocytes, and macrophages, thereby influencing the course of inflammation.
However, the number of neutrophils, the most important cells in the inflammatory response, decreased in the absence of androgen receptors. When the androgen receptor concentration is sufficient, testosterone binds to the androgen receptor in an androgen receptor-dependent manner, leading to an increase in neutrophil production and initiating an inflammatory response in the body. However, the role of testosterone in neutrophil production is highly controversial and requires further substantiation of possible alternative mechanisms. The effect of testosterone on monocytes and macrophages may antagonize the effect of testosterone on neutrophils. Therefore, we believe that the specific role of testosterone may be individualized in terms of the receptor-dependent effects of testosterone on inflammation (44).
As mentioned earlier, the pathogenesis of IBD may involve dysregulation of the body's gut microbiota, hyperimmunity, and inflammatory response, and we speculated that the effect of testosterone on the pathogenesis of IBD may also be related to all three factors to some extent. For the final effect, it may be necessary to consider the different effects of the gut microbiota, inflammation, and immunity together.
In previous studies, testosterone has mostly been recognized clinically as alleviating the progression of some chronic diseases (45). One of the means of treating IBD is antitumor necrosis factor therapy, and testosterone has been shown to suppress immunity (42, 46), which plays a positive role in the treatment of chronic disease. Reduced testosterone levels have been reported in some pediatric IBD patients, and bone as well as physical growth and development have been affected (12), which is a result of multifactorial effects, but it is undeniable that testosterone must play a role in this process. Mason A et al. (40) reported that in most cases, testosterone was beneficial for adolescent boys with IBD who had developmental problems. O'Toole A et al. (47) reported that the use of certain medications or chronic inflammation may lead to hypogonadism, that some patients with IBD can develop metabolic bone disease secondary to hypogonadism (48, 49) and that testosterone replacement has been shown to ameliorate metabolic bone disease in patients with IBD. Many clinical studies have demonstrated that testosterone therapy for IBD patients appears to be effective and safe (48, 50, 51). However, its possible role in disease regression has yet to be proven.
These results were inconsistent with the results of our two-step Mendelian analysis, in which testosterone tended to be a positive factor, but in our analysis, testosterone was a risk factor for the clinical management of IBD. Klil-Drori AJ et al. (52) reported in an observational study that the incidence of IBD was reduced after androgen deprivation in older men with prostate cancer, and the gut microbiota of male mice also more closely resembled that of female mice after depopulation. Therefore, we may be able to hypothesize that androgens have a negative effect on IBD, but the number of clinical studies supporting this conclusion is small. The results of our Mendelian analyses contradicted the currently prevailing belief that testosterone can be used to treat IBD, a finding that has some clinical significance and may provide new guidance for the use of testosterone in the clinical setting.
For the first time, the role of testosterone in IBD was investigated using MR, which was similar to the findings of a randomized controlled study and overcomes the potential limitations of observational studies. Our study used testosterone as an exposure factor and IBD as an outcome. We used different MR analysis methods and obtained largely consistent conclusions, which confirmed that our results were robust and reliable. Moreover, the GWAS database we used (https://gwas.mrcieu.ac.uk/) contains data for 2021 and works perfectly with the two-sample MR package, which is very convenient.
However, the GWAS database we utilized a sample of Europeans. This association with Europeans may not apply to other populations, such as Asians. Therefore, there is a need to explore their causal relationships with other populations in the future.
The onset of IBD may be sex-related, there may be differences between patients with CD and patients with UC (53, 54), and there may be differences in testosterone levels in patients with IBD at different times of life (55). In the future, causal relationships between T and IBD could be explored in other populations, and experimental studies could be used to explore the mechanisms underlying the observed associations. On the other hand, it might be possible to refine the causal relationship between testosterone and IBD into a causal relationship between testosterone and CD and UC, or in the case of studies that categorize female patients according to premenopausal and postmenopausal periods, it might be possible to gain new insights and provide new ideas for future clinical treatment.